In recent years, high-capacity secondary batteries have been desired with the spread of portable telephone terminals and the research and development of electric vehicles and hybrid electric vehicles to cope with environmental issues. As such a secondary battery, a lithium ion secondary battery has already been widely spread, but a technique of using flame-retardant glymes as an electrolyte solution has been proposed in order to secure safety for mounting on a vehicle (for example, Non Patent Literature 1). Further, a technique has been proposed in which an electrolyte solution prepared in a mixing ratio of a Li salt to glyme of 0.70 to 1.25 on a molar basis is used as an electrolyte solution of a lithium secondary battery, wherein a part of the Li salt and the glyme is allowed to form a complex to thereby improve electrochemical stability (for example, Patent Literature 1).
On the other hand, a lithium-sulfur battery has attracted attention as a secondary battery having a higher capacity than a lithium secondary battery (for example, Patent Literatures 2 and 3). Sulfur has a theoretical capacity of about 1670 mAh/g, which is about 10 times higher than the theoretical capacity of LiCoO2 (about 140 mAh/g) which is a positive electrode active material of a lithium battery, and sulfur also has an advantage of low cost and being rich in resources.
With respect to the lithium-sulfur battery, a technique of using an electrolyte solution prepared in a mixing ratio of a Li salt (LiCF3SO3) to tetraglyme of about 0.12 to 0.25 on a molar basis (LiCF3SO3 is 0.5 to 1 mol/L) (for example, Non Patent Literatures 2 and 3); a technique of using an electrolyte solution prepared in a mixing ratio of an alkali metal salt (LiTFSA or the like) to glyme of 0.50 or more on a molar basis by the present inventors (Patent Literature 4); and the like are disclosed.